Implantable cardioverter/defibrillators (ICDs) have been under development since at least the late 1960's. Such a device is described in U.S. Pat. No. Re. 27,757 to Mirowski. A more advanced system is disclosed in U.S. Pat. No. 5,007,422 to Pless et al. which is assigned to the assignee of the present invention and which is incorporated herein by reference. While the technology of ICDs has advanced significantly, a need still exists for improved sensing.
As used herein, the term defibrillation generally may be defined as the correction of either ventricular tachycardia or ventricular fibrillation by the discharge of electrical energy into the heart (0.1-40 joules when discharged through internal electrodes). Ventricular tachycardia is an abnormally rapid heart rate (120-180 beats per minute) originating in the ventricles which is generally regular in periodicity and oftentimes is life threatening to the patient. Ventricular fibrillation is generally a more rapid heartbeat disorder, disorganized and irregular, or non-periodic, and is fatal unless corrected within minutes. This can be accomplished by the discharge of electrical energy through the heart.
Detection and incorrect diagnosis of atrial fibrillation or supraventricular tachycardia by ICDs is a frequent cause of inappropriate shocks. These are heart rhythms generated in the atria which generally do not require defibrillator shocks for treatment. Methods which have been devised to reject these rhythms are often too complex to be incorporated in ICDs using currently available technology, or compromise sensitivity to ventricular tachycardia or ventricular fibrillation. Addition of atrial sensing to an ICD could significantly improve specificity of ventricular tachyarrhythmia detection.
Many different types of electrode systems have been suggested over the years. The above mentioned Mirowski patent describes an electrode arrangement whereby one electrode is formed on the distal end of an intravascular catheter that is positioned within the right ventricle and a second electrode is positioned on the surface of the chest or sutured under the skin of the chest wall or directly to the ventricular myocardium. Mehra et al. disclose a system in U.S. Pat. No. 4,953,551 wherein a first catheter mounted electrode is located in the apex of the right ventricle (RV) and a second electrode carried on the same catheter is located in the superior vena cava (SVC). For some patients, an additional electrode is required to reduce the defibrillation threshold to a safe level. This electrode is a patch electrode which is located subcutaneously outside the chest cavity. This subcutaneous patch electrode is typically provided when the arrangement of the RV electrode and the SVC electrode has a defibrillation threshold (DFT) at the time of implant which is too high to be considered safe and effective. A similar system is described in U.S. Pat. No. 4,662,377 to Hellman et al. Still other systems provide a separate catheter for the SVC electrode along with the catheter for the RV electrode and the subcutaneous patch electrode.
Many defibrillation lead systems include a pacing electrode on the distal tip of the catheter, distal to the RV electrode. In this configuration, the distal tip electrode and the RV electrode are sometimes paired for pacing and sensing functions but are electrically isolated for cardioversion and defibrillation. U.S. Pat. No. 4,603,705 to Speicher et al. describes such a lead. On the other hand, a separate ring electrode may be provided to be paired with the pacing tip electrode. U.S. Pat. No. 5,209,229 to Gilli describes an atrial J and a ventricular endocardial lead, each including tip and ring pacing electrodes and a braid cardioverting electrode.
U.S. Pat. No. 5,243,980 to Mehra, which is assigned to Medtronic, Inc., Minneapolis, Minn., discloses a lead system including a lead for use in the coronary sinus (CS) having two ring electrodes located for positioning within the atrium and intended for use in sensing and stimulating the atrium. According to an article by Bardy et al., entitled "Implantable Transvenous Cardioverter-Defibrillators," in Circulation, April 1993, vol. 87, no. 4, pp. 1152-1168, in five out of fifty patients (10%) receiving a CS lead provided by Medtronic, Inc., the lead dislodged; in three of the five patients, the CS leads would not stay in position despite reapplication. While this dislodgment rate may reflect the early phase of the physician learning curve, or the infancy of the lead design, it is much higher than the lead dislodgment rate of dual-chamber pacemakers (1-4%, as stated in the same article), which typically have one electrode in the right ventricle and one in the right atrium.
Minimizing the number of leads required by including atrial sensing electrodes on the defibrillator lead would reduce the implantation time and the amount of hardware residing in the patient's veins and heart. Also, especially for leads having silicone rubber insulation, placing a second lead often disturbs the placement of the first, since the lead body of the second lead tends to stick to the lead body of the first and drag it along. The problem is worse as more leads are added. Additionally, implantation is simplified with a straight lead having no branching lead body or electrodes.